Recent studies have suggested that in addition to the absolute duration of the QT interval, the dispersion of the QT interval as measured by differences in the QT interval in different leads on a standard 12 lead ECG may be associated with an adverse prognosis in patients at risk for heart rhythm disturbances. Although several population studies have examined QT dispersion, the effects of aging on this potentially important phenomenon have not been described. The purpose of this study is to examine the effects of aging on the QT interval and QT dispersion in both genders before and after autonomic blockage. Gender differences in the raw QT interval with aging have been described but the interaction of gender and aging on QT dispersion are unknown. Forty young and 40 elderly subjects without evidence of structural heart disease will be evaluated in the General Clinical Research Center at Northwestern Memorial Hospital. Subjects will undergo a screening visit to ensure the absence of structural heart disease or systemic diseases which would contraindicate autonomic blockage. On a subsequent visit, subjects will undergo routine 12 lead ECG and body surface maps at several stages: (1) At rest; (2) During staged bicycle exercise testing; (3) After the first drug of a double autonomic blockage protocol (atropine or propranolol); (4) After the second drug (propranolol or atropine); (5) During exercise testing after the administration of double autonomic blockade. The QT interval will be measured in the standard fashion. QT dispersion on the 12 lead EKG will be determined by both manual and computerized methods using algorithms that have been previously developed. QT dispersion on body surface maps will be analyzed using custom designed algorithms which will be developed as part of the study. Repeated measures of analysis of variance will be used to examine differences in the absolute QT interval and in QT dispersion between the young and elderly before and after autonomic blockade. In addition, gender effects will also be examined. Since the elderly are at high risk for serious arrhythmias, understanding the evolution of myocardial repolarization as aging occurs as a physiologic process may have important implications for the problem of sudden cardiac death in the elderly. If the hypothesis that aging alters QT dispersion if verified, future studies will attempt to evaluate the mechanism and clinical implications of this change in myocardial repolariztion with aging.